Influenza A viruses are classified into subtypes on the basis of two surface antigens: hemagglutinin (H) and neuraminidase (N). Three subtypes of hemagglutinin (H1, H2, and H3) and two subtypes of neuraminidase (N1 and N2) are recognized among influenza A viruses that have caused widespread human disease. Immunity to these antigens—especially to the hemagglutinin—reduces the likelihood of infection and lessens the severity of disease if infection occurs. Infection with a virus of one subtype confers little or no protection against viruses of other subtypes. Furthermore, over time, antigenic variation (antigenic drift) within a subtype may be so marked that infection or vaccination with one strain may not induce immunity to distantly related strains of the same subtype. Although influenza B viruses have shown more antigenic stability than influenza A viruses, antigenic variation does occur. For these reasons, major epidemics of respiratory disease caused by new variants of influenza continue to occur.
Young children are at high risk for influenza-related disease and complications. Children <6 months of age have the highest rates of hospitalization and medically-attended illnesses of any age group, but there is no licensed influenza vaccine for this age range. It is known in the art that immunogenicity of flu vaccines is inconsistent and generally poor in infants younger than 6 months (Groothuis et al. Immunization of high-risk infants younger than 18 months of age with split-product influenza vaccine. Pediatrics. 1991; 87: 823-828; Groothuis et al. Immune response to split-product influenza vaccine in preterm and full-term young children. Vaccine. 1992; 10: 221-225). The present invention provides compositions and methods for using such compositions to successfully immunize children younger than six months against influenza.